Abstract. -- Progomphus obscurus Rambur is a burrowing dragonfly species which is abundant in eastern Texas sandy streams. The naiads and
adults of P. obscurus were collected from and around Harmon Creek
(Walker County, Texas) from November 1995 through May 1997. This species
has a univoltine life cycle and produces a total of 11 instars. At
Harmon Creek, emergence of adults began in mid April and continued until
mid to late September. Oviposition was observed from early May through
mid August. First instar naiads were collected from May through early
September. May, June and July were the months of greatest recruitment.
Penultimate naiads were first collected during late February. The annual
secondary production estimate for P. obscurus was 6.842 g/[m.sup.2]/yr,
the standing stock biomass was 1.682 g/[m.sup.2], and the cohort
production/biomass ratio (P/B ratio) was 4.067. The primary food items
consumed by naiads of P. obscurus were chironomid larvae, followed by
mayfly naiads of the families Caenidae and Baetidae.

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Progomphus obscurus Rambur is a burrowing dragonfly species, which
ranges over much of the southern United States. In general, dragonfly
naiads of the genus Progomphus are burrowers found primarily in shifting
sandbars of both lotic and lentic habitats (Needham & Westfall
1955). Analysis of microhabitat preference of P. obscurus naiads
indicates a preference for areas of sand with particle sizes ranging
from 0.625-1.0 mm (Huggins & DuBois 1982). Their front and middle
legs are used for burrowing into the sand, and they burrow completely
beneath the surface, although never deeper than 2 cm (Huggins &
DuBois 1982).

In general, odonates are of tropical evolutionary origin and have
evolved different life cycle patterns and cold resistant stages as they
adapted to temperate regions (Norling 1984). These adaptations produced
two general dragonfly life history patterns described by Corbet (1954)
as spring species and summer species. These two patterns characterize
many dragonfly species, but a continuum of life history patterns between
the extreme spring and summer species has been found, especially in
southern temperate latitudes (Paulson & Jenner 1971). Life history
patterns can also vary within species at different latitudes. For
example, Kormondy & Gower (1965) found Epitheca cynosura to be
semivoltine (bivoltine) in Pennsylvania, while Benke & Benke (1975)
found the same species to be a univoltine spring species in South
Carolina. Life cycles can also vary among individuals within the same
stream. Ferreras-Romero (1997) found Boyeria irene to be mainly
semivoltine, but some naiads required three years to complete
development in the Sierra Morena Mountains of Spain.

While life history patterns have been worked out for some odonate
species, the life cycle, food habits, and production of P. obscurus is
still relatively unknown. Therefore, the objectives of this study were
to elucidate the general life history pattern of P. obscurus, gather
information about food habits of the naiads, and to estimate P. obscurus
production in Harmon Creek.

MATERIALS AND METHODS

Naiads of Progomphus obscurus were collected from Harmon Creek in
Walker County, Texas. The research site was a section of stream with a
sandy bottom passing through eastern Texas piney woods. Monthly
collections of 20 Surber samples were made from November 1995 through
May 1997. Adults were collected during the same period using an aerial
insect net. Both adults and naiads were preserved in 5% formalin.

The head capsule width (HCW) of naiads was measured at the widest
point to the nearest 0.01 mm using a dissection microscope with an
ocular micrometer. Specimens were then dried at 55[degrees]C for 24
hours and weighed. Head capsule width and body weight data were then
In-transformed and a linear regression analysis of body weight on head
capsule width was performed.

Because P. obscurus does not have a tightly synchronous
development, production was estimated using the size-frequency method
originally described by Hynes (1961) and modified by Hamilton (1969) and
Benke (1979). Standing stock biomass was measured, and
production/biomass (P/B) ratio was calculated.

Food habits were determined by stomach content analysis. Stomachs
of both naiads and adults were dissected and their contents removed.
Food organisms contained within each stomach were then identified to the
lowest taxonomic level possible, by using either a dissecting
microscope, or if higher magnification was necessary by mounting the
prey item on a slide for identification using a compound microscope.

The number of instars of P. obscurus was determined by a
combination of two methods. For the first method, which was used to
determine the number of early instars, eggs were collected from
copulating females that were captured in the field. The collected eggs
were incubated in the laboratory at 22[degrees]C until they hatched.
After hatching, each naiad was placed in a separate petri dish, where
the exuviae were collected after each molt. Larvae were raised until
they reached the 6th instar. The size-frequency method was the second
method used to determine the number of instars of P. obscurus. This
method was not very effective for separating early instars (1st through
5th), but was very effective for identifying later instars (6th through
11th). A [chi square] test was used to determine if there were
significant differences in sex ratios of cast exuviae and adults
collected at streamside.

RESULTS

Progomphus obscurus primarily exhibits a univoltine life cycle;
first instar naiads were collected from May through early September at
Harmon Creek. May, June, and July were the months of greatest
recruitment. Penultimate naiads were first collected during late
February. Emerging adults were first observed on 13 April 1996 and on 20
April 1997, which was also the first days that exuviae of penultimate
naiads were collected. Exuviae were last collected on 2 October 1996.
Adults were observed in the area from mid April until late October, with
oviposition occurring from early May through late August.

Just prior to emergence, naiads crawled onto sandy beach areas, dug
their claws into the sand to anchor themselves, and began to emerge. The
distance traveled from water's edge to emergence spot ranged from 5
to 48 cm, with a mean distance of 19 cm. Head capsule width of cast
exuviae ranged from 4.49 mm to 5.02 mm, with a mean width of 4.66 mm (n
= 358).

Incubation time for eggs collected in the field and hatched in the
laboratory ranged from 8 to 12 days. Mean head capsule widths for
laboratory raised 1-5 instar naiads are shown in Table 1. The
combination of data from laboratory-raised naiads and from size
frequency distributions done on field-collected naiads indicates that P.
obscurus produced 11 instars in Harmon Creek (Table 1 and Fig. 1).

The primary food items consumed by P. obscurus naiads at Harmon
Creek were chironomid larvae, followed by mayfly naiads of the families
Caenidae and Baetidae. Small numbers of rotifers, ceratopogonids, and
other odonates, were occasionally ingested. Chironomids were the primary
food items for all P. obscurus size classes, but larger larvae had
larger numbers of mayflies in their diets. During the months of May
through September, greater numbers of mayflies were consumed than during
other months. The number of individuals with food in their stomachs was
variable throughout the year. The fewest individuals with food in their
stomachs (<30%) were found during the months of November through
February. During March, September and October 37-42% of individuals had
food in their guts; Whereas, between the months of April and August over
60% of P. obscurus individuals had food in their guts.

Linear regression analysis showed a significant relationship
between In HCW and In dry weight (W) (F = 278.63, P = 0.0001, [R.sup.2]
= 0.952). The linear relationship between HCW and dry weight can be
explained by the equation: In W = -7.613 + 2.791 (ln HCW). The standard
errors for the Y intercept and the slope were 0.200 and 0.167,
respectively.

[FIGURE 1 OMITTED]

The annual production estimate for P. obscurus was 6.842
g/[m.sup.2]/yr, the standing stock biomass was 1.682 g/[m.sup.2], and
the cohort production/biomass ratio (P/B ratio) was 4.067 (Table 2).

DISCUSSION

Differences in emergence times of P. obscurus are probably related
to the time that the eggs were oviposited. For example, if a naiad
hatched from the egg in June, it would emerge as an adult the following
June; if the naiad hatched in August it would emerge as an adult the
following August. However, final instar naiads begin to appear in late
February, but do not emerge until April; these naiads then spend part of
the month of February, March and then part of April in the final instar.
This indicates that naiads may mature to the final instar in less than a
complete year, but do not emerge immediately. Possibly, low water
temperatures corresponding with low air temperatures delay the emergence
process until the likelihood of freezing temperatures is very low.

The life history pattern of P. obscurus is quite different from
that of the gomphid, Onychogomphus uncatus, in France (Schutte et al.
1998). In this French study it was found that O. uncatus had a three
year life cycle which is much longer compared to the one year cycle for
P. obscurus; This may be partially explained by O. uncatus usually
producing 13 instars compared to the 11 of P. obscurus. The closely
related gomphid in Florida, Progomphus bellei, produced mature larvae
during March and emergence began in mid-April (Knopf & Tennessen
1980). This development time is similar to what was observed in this
study. Much different from P. obscurus was the gomphid Lanthus vernalis,
which showed mixed voltinism and lacked a clear growth pattern of its
naiads, but was at least semivoltine in a cold unproductive stream in
South Carolina (Folsom & Manuel 1983).

Voltinism, emergence periods, and adult flight periods are quite
variable for odonates in general. Spring species, as described by Corbet
(1954), emerge synchronously during the spring over a short period of
time. The synchronous emergence occurs because these species spend the
winter before emergence in the final instar (some in a diapause). Summer
species spend the winter before emergence in earlier larval instars and
then emerge asynchronously throughout the summer. Corbet (1964) noted
that the degree of synchrony of emergence is inversely proportional to
the number of overwintering instars in the winter preceding emergence.
As observed by Benke & Benke (1975), the synchronous or asynchronous emergence patterns reflect a similar type of development (i.e. spring
species develop synchronously and summer species develop
asynchronously). Progomphus obscurus fits best into the category of
summer species proposed by Corbet (1954). The emergence period of P.
obscurus extends from mid April until mid September, with a
correspondingly long flight period, but gomphids in general can fit into
either life history pattern. In a stream in Spain, Onychogomphus uncatus
had a brief early spring emergence period consistent with the spring
pattern (Ferreras-Romero & Corbet 1995). Suhling (1995) studied two
different populations of O. uncatus in the southern part of France and
found that one showed the emergence pattern of a summer species and the
other showed the spring species pattern. This indicates that a single
species can have different life history patterns in different habitat
types within the same geographic region. The flight period of P.
obscurus at Harmon Creek from mid April until late October is longer
than that observed in more northern areas of the U.S., such as North
Carolina where the flight period extended from April through September
(Paulson & Jenner 1971). Even farther north in the U.S. in Indiana,
the flight period is reduced further to May through mid October (Bick
1941).

Incubation time for P. obscurus eggs collected from adults and
reared in the laboratory ranged from 8-12 days, which is longer than 5-7
day incubation time for the libellulid Tramea lacerata (Bick 1951). Bick
(1941) determined that the incubation period of another libellulid
species, Erythemis simplicicollis, varied from 10 to 16 days.

Laboratory studies of odonates have shown that they will feed on a
variety of animals including Paramecium sp. and other protozoans, Culex eggs, mosquito larvae, mayfly naiads, and amphipods (Bick 1951). In the
current study, food habits of P. obscurus varied with size class, but
prey were predominantly chironomids for all size classes. Although early
instars were found to feed on rotifers to some extent, the majority of
their prey were still chironomids. Overall, when compared to libellulid
and corduliid species investigated by Benke (1978) food habits of P.
obscurus were similar to those of Ladona deplanata, Epitheca sp. and
Celithemis fasciata which fed mainly on immatures of Chironomidae and
Ephemeroptera (mostly Caenis sp.) and to a much lesser extent on
cladocerans and ostracods. The species studied by Benke (1978) were from
a lentic habitat.

Production has been calculated for only a few species of odonates
(Benke 1976; Dudgeon 1989). In the current study, annual production of
6.842 g/[m.sup.2]/yr seems high for a single species of predaceous invertebrate, and is roughly comparable to the combined production of
three odonate species in the pond studied by Benke (1976). Using the
removal-summation method, Benke (1976) found that production of L.
deplanata (Libellulidae) was 2.20 g/[m.sup.2]/yr, Epitheca sp.
(Corduliidae) was 1.94 g/[m.sup.2]/yr, and C. fasciata (Libellulidae)
was 2.27 g/[m.sup.2]/yr. Dudgeon (1989) estimated production of two
species of gomphid naiads in a Hong Kong forest stream, and found that
production of Heliogompus scorpio was 0.182 g/[m.sup.2]/yr and
production of Onychogomphus sinicus was 0.236 g/[m.sup.2]/yr. Total
odonate production in the stream was estimated to be 1.019
g/[m.sup.2]/yr. The production of P. obscurus in this study was much
higher than that found for either of the gomphids H. scorpio or O.
sinicus, or the total production of odonates in the forest stream study
in Hong Kong (Dudgeon 1989). One reason for the high single species
production of P. obscurus may be that, based on extensive sampling of
Harmon Creek, P. obscurus is the only abundant invertebrate predator
that inhabits the sandy substrate. Also, general trends for comparison
are difficult to make because few estimates of odonate production have
been published.

The sex ratio of P. obscurus exuviae in this study was female
biased, which is typical of the emergence sex ratio of most gomphids
previously studied (Miller 1964; Lutz & McMahan 1973). However,
Suhling (1995) found some discrepancies in the emergence sex ratio of O.
uncatus when sampling two different streams. Based on male biased sex
ratios, it appears as though female P. obscurus mate relatively quickly,
oviposit, and then move to more upland habitats to feed, and that males
remain near the stream to look for more mates. This observation was also
made by Needham & Westfall (1955). This behavior would explain the
discrepancy between sex ratios observed in captured adults versus
exuviae.